Thank you, all. The manual for 9.4 is indeed clearer on this point than the 9.1 version.
Just to nit-pick, I see nowhere in either version of the manual the indication that it is normal for postgresql to continue to update files in its data catalog between pg_start_backup and pg_stop_backup. The closest I see comes in this paragraph:
** Some file system backup tools emit warnings or errors if the files they are trying to copy change while the copy proceeds. When taking a base backup of an active database, this situation is normal and not an error.
Does "this situation" refer to the tools emitting warnings or to the fact that postgresql is updating the files? It might be the case, for instance, that timestamps are updated but not the contents of the files (this is what I had assumed prior to today).
